Although adhesives, adhesion-promoters, and adhesion-enhancing treatments have been developed to meet a tremendous number of varied applications, 1,2 designing systems in which adhesion itself is responsive to changes in environmental conditions would offer opportunities for active control of adhesion over time (e.g., in applications such as antifouling, barrier films, or cell adhesion). 3 We report here a polymer/metal interface that displays reversible, temperature-dependent adhesion arising from rubber elasticity in the interfacial region of the polymer. Our strategy for coupling rubber elasticity and interfacial adhesion involved constructing a system that required extension of polymer chains out of randomcoil conformations to bring adhesion-promoting functional groups to the polymer/metal interface. As a result, the entropic loss associated with chain extension would provide a restoring force for removing these groups from the interface. The opposition of this entropic force to enthalpically favorable chemical interactions at the polymer/metal interface would, in turn, provide the temperature dependence of adhesion.Oxidation of the surface of cross-linked 1,4-polybutadiene (1,4-PBD) with aqueous permanganate introduces carboxylic acid and other functional groups 4,5 within the interfacial region. Although polymer surfaces usually reconstruct against water to maximize hydrogen bonding at the interface, 6,7 we have demonstrated previously that this surface-modified elastomer reconstructs reversibly against water as a function of temperature, to produce a hydrophilic surface (hydrogen-bonding groups in contact with the water) at low temperature but a hydrophobic one at higher temperature. 4 We expected this material to behave analogously against aluminum because of the well-established affinity of carboxylic-acid groups for the native oxide of this metal. 7 Cross-linked polymer films were prepared by mixing 1,4-PBD (36% cis, 55% trans, and 9% 1,2-; M w ) 420 000 g/mol) with 0.02 phr of dicumyl peroxide and curing at 150°C under 125 psi for 84 min (8 half-lives). 8 The cured film (approximately 1 mm thick; M c ≈ 30 000 g/mol) was cut into strips having lateral dimensions of ∼10 cm × 1 cm. The unbound fraction -polymer chains not incorporated into the network -was extracted by swelling the strips in toluene. 4 This step ensured that reconstruction at the polymer surface would be due only to the movement of chains that were part of the cross-linked network. One of the broad surfaces on each sample was oxidized with a basic aqueous solution of KMnO 4 for 50 min at room temperature, as described previously, 4 to introduce carboxylic acid and other functional groups within the interfacial region. As a result of this oxidation, the contact angle of water (pH 1) dropped from 84-88°to 66-72°. 10 Aluminum strips (99.5%; thickness ∼0.007 mm) were cut to dimensions of ∼13 cm × 1 cm and sonicated in 75 mL of 3% (v/v) aqueous detergent (Detergent 8; VWR) for 0.5 h. 11 They were then thoroughly rinsed with deionized water an...
Oxidation of the surface of cross-linked 1,4-polybutadiene provided a hydrophilic substrate that reconstructed against hot water to become more hydrophobic. Subsequent equilibration against water at room temperature returned its original hydrophilicity. These temperature-dependent changes in the relative concentrations of hydrophobic and hydrophilic moieties at the polymer/water interface are interpreted as arising from the entropic influence of chain extension, associated with rubber elasticity. As expected, the magnitude of this effect depended on the cross-link density of the polymer and the degree of oxidation of the surface. The reversibility of the reconstruction when the water was cycled between high and low temperature damped out only gradually over many cycles.
Surface modification of 1,4-polybutadiene and cis-1,4-polyisoprene to introduce polar functional groups provided surfaces that reconstructed reversibly against water as a function of temperature. These surfaces became hydrophobic in contact with hot water, but their original hydrophilicity returned upon equilibration against cold water. Repeated cycling between hot and cold water, however, led to a damping of this reversibility. A series of parallel experiments on both the interfacial and bulk behavior of these elastomers strongly indicated that this damping was due to the alignment of extended interfacial chains during temperature cycling and to a decay of the restoring force on the interfacial chains under extension. These studies thus demonstrate that the interfacial behavior of elastomers can display close analogies to the bulk viscoelastic properties of the solid.
After oxidation of the surface of cross-linked cis-1,4-polyisoprene with aqueous permanganate, the composition of its interface with water depended on temperature. The polymer remained hydrophilic against cool water but became more hydrophobic against hot water. These changes depended upon the amount of cross-linking and on the amount of surface oxidation. They were also reversible when cycled between high and low temperature, and this reversibility disappeared only gradually over many cycles. The system independence of this behavior provided an important test of our hypothesis that the surface dynamics of elastomers can involve a competition between enthalpically favorable solvation of hydrophilic functional groups at the polymer/water interface and entropically unfavorable chain extension.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.